1. Field of the Invention
This invention relates to front suspension and steering systems for motorcycles as a more practical and more efficient system than the presently used and accepted fork system.
2. Background and Related Art
All accepted modern high-performance motorcycles use telescopic forks. Although fork development has kept pace with motorcycle needs, real problems remain in motorcycle front syspension design. Such problems as speed of steering response and geometry change are critical and a compromise. Further, with a steered fork unsupported at the wheel centre, a motorcycle's front resembles a mechanical oscillator, or pendulum, and like a pendulum has a natural frequency. When subjected to any force, the system will wobble. Adequate damping will absorb this wobble (or oscillation), but with increasing energy entering the system oscillation builds up faster than damping can eliminate it. Further, to compound problems, unsupported front forks constantly subject to immense forces, bend and twist dramatically.
Alternative systems have been attempted but either offer no advantage over the single fork or are too complicated or impractical in concept. Typical examples of these are true centre hub systems of Difazio (The Nessie) which uses an upper arm to hold rake and utilizes drag-link steering; alternatively, the Bimoto Tesi uses upper arms to hold rake angle inside the hub and steering is obtained through hydraulics. On systems like the Tesi, large wheel bearings surround a very small steering pivot--which is the opposite of what is required. The steering pivot demands very high tolerances and is prone to wear, which can have dramatic consequences and be extremely expensive. Their production possiblities would be impractical.
Other systems such as the Elf use a pair of forward projecting single-sided arms, the forward ends being ball-jointed to carry a steering upright from the side of which projects the front wheel spindle. The wheel is deeply dished to allow the ball joints to be situated in the wheel's central plane. With the steered wheel supported from one side by cantilever arms, rigidity is difficult to achieve and of necessity a large trail is required.
The Foale hub-steerer and the Radd U.S. Pat. No. 4,526,249, uses the lower single side arm of the Elf type connecting to the lower end of a steering upright, which in turn is ball-jointed to a reverse wishbone above the wheel. This, like the Elf, is a single-sided swing arm and, while being practical, is nevertheless unbalanced and lopsided, and has little real advantage over the simple fork.
Less radical and likely more workable concepts are the Hossach, Fior, U.S. Pat. No. 4,388,978, and more recently McKagen, U.S. Pat. No. 4,627,632 systems. While these are reasonably practical, the only possible advantage is that the positioning of the two control arms (wishbones) means that the stress fed through them into the frame is less concentrated than in the conventional fork system. They are still prone to the front fork oscillation problem, though to a lesser amount, and to flex, as well as the fact that levers and knuckles are not the most simple or best wearing of steering methods.
In summation, none of the available systems manage to unify all the desired characteristics in a single design. After all, it is critical when considering replacing the fork to approach the basic problem, and that is faster, more stable steering response and a method for a motorcycle to steer out of corners with a rigid progressive front end.